Beta-dicarbonyl compounds for the promotion of aromatic nitration by nitrogen tetroxide

ABSTRACT

Aromatic hydrocarbons are nitrated by reacting the aromatic hydrocarbon in a nitrogen tetroxide-containing liquid reaction medium, in the presence of a beta-dicarbonyl compound having a &#34;W&#34; configuration with a carbonyl group at each apex of the &#34;W&#34;, preferably at a pressure and temperature sufficient to maintain the nitrogen tetroxide in the liquid state. Addition of an inert organic co-solvent to the liquid nitrogen tetroxide for the metal acetylacetonate promoted nitration of benzene essentially eliminates the production of dinitrobenzenes.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is related to our co-filed application titled "Solvent Modification of Nitrations By N₂ O₄ /Metal Acetylacetonate Systems," Ser. No. 386,857, filed June 10, 1982, now U.S. Pat. No. 4,417,080, which is incorporated by reference.

TECHNICAL FIELD

This invention relates to a process for the nitration of aromatic hydrocarbons. More particularly, this invention is concerned with an improved process for the selective nitration of aromatic hydrocarbons by nitrogen tetroxide.

BACKGROUND OF THE INVENTION

The nitration of aromatic hydrocarbons, and, in particular, of benzene and toluene, is normally carried out on a commercial scale by reacting the aromatic hydrocarbon with a mixture of nitric and sulfuric acids under controlled temperature. This mixed acid process suffers from the use of large volumes of the highly corrosive mixture of nitric and sulfuric acids and the formation of by-products including the highly undesirable and potentially explosive polynitrated hydrocarbons.

Various methods for nitrating aromatic hydrocarbons without the use of a mixed acid are known:

U.S. Pat. No. 1,325,168 describes passing an oxidizing agent, such as oxygen, into a mixture of an aromatic hydrocarbon and liquid nitrogen peroxide.

U.S. Pat. No. 2,109,873 describes passing a gaseous mixture of an aromatic hydrocarbon and nitrogen dioxide through a bed of silica gel to obtain the mononitro derivative of the aromatic hydrocarbon.

U.S. Pat. No. 2,431,585 describes the vapor phase nitration of an aromatic hydrocarbon to produce the mononitro derivative using nitrogen dioxide and a catalyst which is selected from metal metaphosphate, boron phosphate, solid and supported phosphoric acid.

U.S. Pat. No. 3,459,816 describes nitrating a dialkylbenzene with dinitrogen tetroxide in a halogenated hydrocarbon solvent at a temperature of about 90° to 160° C. to effect substitution of the nitro group on the alkyl substituent.

U.S. Pat. No. 3,856,859 discloses the position-selective nitration of alkanesulfonanilides using an equimolar amount of a nitrating agent, such as dinitrogen tetroxide in chloroform or trifluoroacetic acid.

U.S. Pat. No. 3,922,315 describes a process for the mononitration of aromatic compounds by contacting the aromatic compound with nitrogen dioxide and/or its dimer, nitrogen tetroxide, in the presence of a rhodium catalyst and hydrogen.

U.S. Pat. No. 3,929,917 discloses a process for mononitrating ortho-xylene by reacting the ortho-xylene with nitrogen tetroxide (N₂ O₄) or nitrogen dioxide (NO₂) and an oxygen containing gas in the presence of a mercuric salt dissolved in acetic acid at temperatures above about 20° C.

U.S. Pat. No. 4,028,425 describes a continuous process for the concurrent production of dinitrotoluene and nitric acid by continuously introducing toluene, nitrogen dioxide or nitrogen tetroxide and oxygen in specified molar ratios into a reaction zone containing a reaction medium comprising dinitrotoluene and 93-103% nitric acid.

U.S. Pat. No. 4,104,145 describes the nitration of benzene by the use of nitrogen dioxide under irradiation of visible ray or ultra violet ray in the presence of oxygen.

U.S. Pat. No. 4,123,466 describes reacting an aromatic hydrocarbon with gaseous nitrogen dioxide in the presence of a catalytic amount of sulfuric acid and in the absence of oxygen.

D. S. Ross and W. G. Blucher, "A Novel Nitration System," 174th meeting of Am. Chem. Soc., Chicago, Il., Aug. 1977 disclosed the production of nitrobenzenes by dissolving benzene and nitric oxide (NO) in liquid N₂ O₄ at 0° C. under 2 atmospheres oxygen.

SUMMARY OF THE INVENTION

In its broadest aspect, the invention comprises reacting an aromatic hydrocarbon with nitrogen tetroxide (N₂ O₄) in a liquid reaction medium containing an effective amount of a beta-dicarbonyl compound which can assume a "W" orientation having a carbonyl group at each apex, at a temperature sufficient to effect nitration, preferably under a pressure which is sufficient to maintain the nitrogen tetroxide in the liquid state at the reaction temperature.

Compared to prior art liquid N₂ O₄ reactions, an advantage of this invention is the substantial suppression of phenolic by-product formation. Another advantage of the invention is the reduction in the degree of dinitration of the aromatic hydrocarbons, i.e. the inventive process is selective towards the production of mononitrated aromatic hydrocarbons. A further advantage is the elimination of the use of an acidic material, such as aqueous sulfuric acid, acetic acid, trifluoroacetic acid and the like, in the nitration reaction medium.

DETAILED DESCRIPTION OF THE INVENTION

The invention relates to a process for the nitration of an aromatic hydrocarbon and comprises reacting the aromatic hydrocarbon with N₂ O₄ in a liquid reaction medium containing an effective amount of a beta-dicarbonyl compound, which compound can structurally orient itself into a "W" having a carbonyl group at each apex, at a temperature sufficient to effect reaction between the aromatic hydrocarbon and the N₂ O₄, and preferably under a pressure sufficient to maintain the N₂ O₄ essentially as a liquid at the reaction temperature.

Suitable as aromatic hydrocarbon reactants that can be used in practicing the process are benzene, toluene, naphthalene, anthracene and their alkyl or alkoxy substituted derivatives, and other benzenoids, as well as non-benzenoid aromatic hydrocarbons. Benzene and toluene are especially preferred reactants.

The reaction medium may comprise liquid nitrogen tetroxide which is an equilibrium mixture of NO₂ and N₂ O₄ having a melting point of about -11° C. and a boiling point of about 21° C. at atmospheric pressure. Although only a limited amount of N₂ O₄ is required for each mole of aromatic hydrocarbon to afford the mononitro derivative, liquid N₂ O₄ is advantageously used in considerable excess.

Desirably, an inert organic solvent may be added to the liquid nitrogen tetroxide as a co-solvent. Co-solvents are added to improve the solubility of the beta-dicarbonyl compound in the liquid N₂ O₄ reaction medium, thus facilitating the nitration of the aromatic hydrocarbon. A co-solvent is especially desirable for metal acetylacetonates. Further, it has unexpectedly been discovered that a liquid nitrogen tetroxide/co-solvent reaction medium containing a metal acetylacetonate substantially eliminates the formation of dinitrosubstituted benzenes. By an "inert organic solvent" is meant a liquid organic compound which does not substantially react with the N₂ O₄, the beta-dicarbonyl promoter or co-products under the conditions of the nitration reaction.

Contemplated as the functional, or operative, equivalent of a liquid N₂ O₄ /inert organic solvent reaction medium for purposes of the invention is the inert organic solvent containing dissolved gaseous nitrogen tetroxide, especially such solvent saturated with gaseous nitrogen tetroxide.

Representative inert organic solvents which can be used as co-solvents in the process of this invention and for the selective production of mononitrated benzene include, for example, nitroaliphatics and nitroaromatics, acyclic and cyclic aliphatics, and halogenated aliphatics. Specific examples of such solvents are nitromethane, nitroethane, nitrobenzene, cyclohexane, methylene chloride, chloroform, carbon tetrachloride, trichlorotrifluoroethane, and the like.

The amount of inert organic solvent used in the practice of the invention with respect to the amount of nitrogen tetroxide, aromatic hydrocarbon or beta-dicarbonyl compound is not critical although it is preferred to use excess quantities, e.g. a volume of solvent about equal to or greater than the volume of liquid nitrogen tetroxide.

Advantageously, the beta-dicarbonyl compound for practicing the process described herein should be soluble in the reaction medium. "Soluble" means that the compound is capable of sufficiently dissolving in the liquid reaction medium to yield a solution that is effective for practicing the inventive process. Needless to say, the more soluble the beta-dicarbonyl compound in the nitration reaction medium the better.

The beta-dicarbonyl compounds useful as promoters for the nitration of aromatic hydrocarbons using nitrogen tetroxide are those beta-dicarbonyl compounds which can assume a "W" configuration having a carbonyl function composing each apex. Suitable beta-dicarbonyl compounds contain a structural unit which can be represented by the formula: ##STR1## where Z represents an oxygen atom or a substituted or unsubstituted methylene group with the unsubstituted methylene group (--CH₂ --) preferred. With regard to substitution on the methylene group, C₁ -C₄ alkyl radicals are preferred.

When Z in the above formula is an oxygen atom, the useful beta-dicarbonyl compounds are acid anhydrides such as, for example, acetic anhydride, propionic anhydride, benzoic anhydride and the like. An example of an unsuitable acid anhydride is succinic anhydride.

When Z is a substituted or unsubstituted methylene group, suitable beta-dicarbonyl compounds include beta-diketones, beta-ketoacids, beta-ketoesters, beta-diacids, and beta-diesters. Representative compounds useful as nitration promoters in the process of this invention are acetylacetone (2,4-pentanedione), acetoacetic acid, benzoylacetic acid, ethyl acetoacetate, methyl benzoylacetate, malonic acid, methyl malonic acid (isosuccinic acid), malonic acid dimethyl ester, methylmalonic acid diethyl ester and the like.

Also useful in the practice of this invention are the metal salts of suitable beta-dicarbonyl compounds having an active hydrogen atom which may be attached to the methylene carbon atom or a carboxylate oxygen atom.

The metal beta-dicarbonyl compounds are prepared by reacting a beta-dicarbonyl compound having such active hydrogen atom with, for example, an oxide, hydroxide, carbonate, hydride or the like of a metal such as an alkali, alkaline earth or transition metal in an appropriate solvent as is well known in the art. Exemplary of suitable metal beta-dicarbonyl compounds are cobalt(III)acetylacetonate, iron(III)acetylacetonate, lithium acetylacetonate, copper(II)acetylacetonate, chromium(III)ethylacetoacetate, sodium malonate, magnesium methylmalonate, potassium diethylmalonate and the like.

The preferred metal beta-dicarbonyl compounds are those compounds in which the metal is a transition metal, particularly the first row transition metals of the Periodic Table, and especially those transition metals having available oxidation states separated by one unit. However, transition metals with oxidation states separated by two or more units may be used in the invention. For example, compounds of titanium, vanadium, chromium, manganese, iron, cobalt, nickel or copper may be used.

It has been found that compounds of the type "metal acetylacetonate" have exceptional solubility in liquid N₂ O₄ and are especially desirable nitration promoters in the inventive process. Metal acetylacetonates may be identified as M(acac)_(n) where M represents a metal ion, n is the oxidation number of the metal ion and (acac) represents the acetylacetonate radical.

The above-mentioned beta-dicarbonyl compounds are not intended to be exhaustive of those which may be employed in the reaction. However, as might be expected, it is preferred to use those which are more reactive and provide for substantial conversion with high selectivity to the desired mononitrosubstituted product.

The quantity of soluble beta-dicarbonyl compound used in the nitration reaction is empirical and can vary widely depending upon the reactivity of the beta-dicarbonyl compound and the reactivity of the aromatic hydrocarbon. An effective amount of a soluble beta-dicarbonyl compound is used; in other words, an amount which causes a reaction between the aromatic hydrocarbon and the nitrogen tetroxide to yield a nitroaromatic hydrocarbon at the temperature and pressure used. Usually, though, the amount used ranges from about 5 to 40 mole % or more based upon the total amount of the aromatic hydrocarbon present in the reaction mixture, and preferably in an amount from about 5 to 10 mole %. When the nitration promoter is a transition metal beta-dicarbonyl compound, it may be used in an amount ranging from 5 to 40 mole %. Within these ranges though, the level of beta-dicarbonyl compound is empirical.

In the preparation of nitroaromatic hydrocarbons the reaction is maintained at a temperature sufficient to effect a reaction involving N₂ O₄ and the aromatic hydrocarbon to yield a nitrosubstituted compound. The temperature is preferably above about -11° C., the melting point of nitrogen tetroxide, to about 21° C., the boiling point of nitrogen tetroxide, when operating at atmospheric pressure. However, temperatures above and below 21° C. may be used with a reaction medium comprising an inert organic co-solvent saturated with gaseous N₂ O₄.

Above 21° C., the vapor pressure of nitrogen tetroxide is greater than one atmosphere, attaining a value of about five atmospheres at 60° C. and twenty atmospheres at about 100° C. Accordingly, temperatures above 21° C. may also be used provided the reaction pressure is sufficient to maintain the nitrogen tetroxide, with or without a co-solvent, in the liquid reaction medium. (As a word of caution, the pressurized reactions above 21° C. present the potential for runaway oxidation and detonation!) Preferably the pressure utilized for carrying out the reaction is that autogenous pressure which is sufficient to maintain the nitrogen tetroxide in essentially liquid phase at the reaction temperature. When utilizing these temperatures and pressures, the reaction is allowed to proceed until a desired conversion is obtained or the reaction is complete. Normally, the reaction is carried out within about 1.5 to 3 hours.

Generally, the amount of nitrogen tetroxide is at least one-half mole per mole of aromatic hydrocarbon compound for each nitro group which is introduced to the aromatic compound. However, with a high ratio of aromatic hydrocarbon to liquid nitrogen tetroxide the dielectric constant of the reactant may modify the nature of the reaction medium. Therefore, it is preferred that the liquid nitrogen tetroxide be present in great excess of that required for the desired degree of nitration.

Although the reaction of the aromatic hydrocarbon will typically be performed in the presence of a great excess of nitrogen tetroxide and substantially selectively affords the mononitro product, it is nevertheless possible to further control the reaction to selectively produce the mononitroaromatic hydrocarbon. This can be achieved by controlling the reaction time and ceasing the reaction before greater than minor quantities of the dinitroaromatic compounds are produced.

After initial product formation occurs, the resultant product is stable in the reaction medium. Prolonged reaction times would not result in product degradation, only in the production of dinitrosubstituted products.

Recovery of the nitro compounds from the reaction mixture can be accomplished by conventional techniques, these techniques generally involving a distillation. Illustratively, the liquid nitrogen tetroxide can be allowed to distill from the reaction vessel and the residue can be subjected to a distillation or crystallization process to yield the desired product.

In general, the nitration process comprises introducing liquid nitrogen tetroxide into a temperature controlled reaction vessel. It is also possible to directly condense nitrogen tetroxide in the reaction vessel. An inert organic co-solvent may be added to the reaction vessel before or after the introduction of the nitrogen tetroxide.

Oxygen is admitted into the liquid nitrogen tetroxide through a feed tube below the surface until any trace of nitrogen trioxide, which is green in color, has been oxidized to nitrogen tetroxide. When this operation is completed and a straw colored liquid which is characteristic of nitrogen tetroxide remains, the oxygen flow is terminated. Nitrogen gas is then bubbled through the reaction mixture to purge excess oxygen from the system. The nitrogen purge of the reaction medium is optional but recommended since it is suspected that oxygen generates phenolic by-products. The beta-dicarbonyl compound is carefully added to the liquid nitrogen tetroxide followed by the aromatic reactant although this order of addition may be reversed. If compatible, the beta-dicarbonyl compound and the aromatic hydrocarbon reactant can be added as one.

In the absence of an organic cosolvent the addition order is important. The powdered metal acetylacetonates usually dissolve safely when slowly dropped into a stirring quantity of nitrogen tetroxide. However, when nitrogen tetroxide is added to several milligrams of the solid metal compounds, a brilliant flame erupts. Needless to say, the former addition mode is recommended, and with care.

It has also been discovered that the nitration proceeds at a faster rate if the aromatic hydrocarbon is added to the N₂ O₄ reaction medium followed by the metal acetylacetonate (fast mode) in contrast to first adding the metal acetylacetonate followed by aromatic hydrocarbon addition (slow mode).

The beta-dicarbonyl promoted nitration reaction may also be performed in a reaction medium comprising gaseous nitrogen tetroxide bubbling into an inert organic solvent, preferably saturating the organic solvent with the gaseous N₂ O₄.

If the reaction is proceeding at atmospheric pressure and a temperature of 20° C. or less, a condenser above the reaction vessel is used to recondense any nitrogen tetroxide vapors. At superatmospheric pressures a pressure reaction vessel, which may be fitted with a reflux condenser to remove the heat of reaction, is employed.

The following examples illustrate the nature of the inventive process and are not intended to limit the scope of the invention.

EXAMPLE 1

For Runs 1-9 nitrogen tetroxide was condensed into a reaction flask cooled to 0° C. As an inert organic co-solvent nitromethane was added to the liquid nitrogen tetroxide for Runs 2-9. The green tinted, liquid nitrogen tetroxide reaction medium was oxidized to a straw color by bubbling oxygen beneath the surface at about 10 scc/min to ensure that the small quantities of NO always present in the N₂ O₄ are oxidized to NO₂. Flow of oxygen was stopped and nitrogen was bubbled through the reaction medium for at least two minutes to remove any dissolved oxygen.

Next, toluene (Runs 1-9) and a promoter (Runs 3-9) were added to the reaction medium maintained at 0° C.

                                      TABLE 1                                      __________________________________________________________________________                  N.sub.2 O.sub.4                                                                    CH.sub.3 NO.sub.2                                                                   Toluene                                                                             % Toluene Converted                                                                       Isomer Ratio                                                                          % MNT with respect                Run                                                                               Promoter (mmole)                                                                         ml  ml   mmole                                                                               to MNT in (min)                                                                           o:m:p  to Promoter                       __________________________________________________________________________     1  None      20  --   60.1 0.9  (101) 53:4:43                                                                               --                                2  None      5   20   60.0 0.1  (109) 62:5:34                                                                               --                                3  Acetic    10  10   60.0 12.1  (66) 59:2:39                                                                               99.1                                 Anhydride                                                                      (7.35)                                                                      4  2,4 Pentanedione                                                                         5   20   60.1 11.7 (104) 61:4:35                                                                               49.5                                 (acetylacetone)                                                                (14.26)                                                                     5  Ethyl     5   20   60.1 10.2 (102) 61:3:36                                                                               51.3                                 Acetoacetate                                                                   (11.99)                                                                     6  Acetic Acid                                                                              5   20   60.1 0.1  (149) 63:3:34                                                                               0.4                                  (14.38)                                                                     7  Succinic* 5   20   60.1 0.4  (108) 69:7:24                                                                               1.8                                  Anhydride                                                                      (14.25)                                                                     8  Diethyloxalate                                                                           5   20   60.2 0.4  (108) 60:5:33                                                                               1.6                                  (14.31)                                                                     9  Dimedone  5   20   60.1 0.3  (145) 62:5:33                                                                               2.3                                  (7.84)                                                                      __________________________________________________________________________      *Most of the anhydrtide anhydride undissolved                            

A small amount of nitrobenzene was also added as an internal standard. Samples were removed periodically and mixed with an equal volume of methylene chloride for gas chromatographic analysis.

Table 1 shows the quantities of reactants and co-solvent and the data for the products of the reaction.

Runs 1 and 2 were background runs using liquid N₂ O₄ and liquid N₂ O₄ /nitromethane (CH₃ NO₂) reaction media, respectively. These runs showed that less than 1% toluene was converted to a mononitrotoluene (MNT) in a little over 100 minutes.

The beta-dicarbonyl promoters acetic anhydride (Run 3), acetylacetone (Run 4) and ethyl acetoacetate (Run 5) showed substantial production of mononitrotoluene after about 100 minutes, i.e. at least about 50% mononitrotoluene production with respect to molar quantity of promoter. Acetic anhydride afforded nearly an equimolar amount of mononitrotoluenes. Acetic acid (Run 6), on the other hand, gave only 0.4% mononitrotoluenes.

Succinic anhydride (Run 7) and dimedone (Run 9), which are beta-dicarbonyl compounds that are not structurally orientable as a "W" having a carbonyl group at each apex, gave only about 0.4% toluene conversion to mononitrotoluene and about 2% mononitrotoluene with respect to promoter.

Diethyloxalate (Run 8), which is an example of an alpha-dicarbonyl compound, gave results comparable to succinic anhydride and dimedone.

EXAMPLE 2

For Run 10 nitrogen tetroxide (20 ml) was condensed into a reaction flask cooled to 0° C. The green tinted nitrogen tetroxide liquid was oxidized to a rich straw color by bubbling oxygen beneath the surface at a rate of 10 scc/min. Flow of oxygen was stopped and nitrogen was bubbled through the solution for two minutes to remove any dissolved oxygen.

Next, 2.79 mmoles of cobalt(III)acetylacetonate were introduced into the reaction vessel. The green solid immediately dissolved in the liquid and a dark red solution was obtained. To the solution was then added 11.26 mmoles of benzene, together with 5.0 mmoles of 4-nitrotoluene as an internal standard for later gas chromatographic analysis. The reaction was maintained at about 0° C. Samples were removed periodically and mixed with an equal volume of methylene chloride for gas chromatographic analysis.

                                      TABLE 2                                      __________________________________________________________________________     RUN 10                                                                                                 % yield NB                                                   Time                                                                              NB   DNB       With respect to                                                                        With respect to                                SAMPLE                                                                               min.                                                                              mmoles                                                                              mmoles                                                                              NB:DNB                                                                              benzene Co(acac).sub.3                                 __________________________________________________________________________     A      20                                                                               0.30 .02  15.0 2.7     10.8                                           B      59                                                                               0.86 .02  43.0 7.6     30.8                                           C      98                                                                               1.71 .04  42.8 15.2    61.3                                           D     130                                                                               2.32 .06  38.7 20.6    83.2                                           E     183                                                                               2.89 .09  32.1 25.7    103.6                                          F.sup.a                                                                              291                                                                               3.36 .11.sup.b                                                                           30.5 29.8    120.4                                          __________________________________________________________________________      .sup.a Phenols <0.04 mmoles                                                    .sup.b o:m:p = 6:67:27                                                   

Table 2 presents the gas chromatography yield data for samples A-F of Run 10 which were taken at various times during the course of the nitration reaction. The quantities (mmoles) of nitrobenzene (NB) and dinitrobenzenes (DNB) are set forth. The percent yield of nitrobenzene with respect to the limiting starting quantity of cobalt(III) acetylacetonate appeared to be approaching a limit after 291 minutes of reaction time. While samples A-F showed a steady increase in nitrobenzene and dinitrobenzenes content, the nitrobenzene:dinitrobenzenes ratio steadily decreased with time, not considering the first sample. Nevertheless, it is readily apparent from the data that the nitration reaction selectively afforded primarily the mononitro substituted product.

EXAMPLE 3

The procedure of Example 2 was followed for Run 11 using 40 ml N₂ O₄ liquid; 5.41 mmoles cobalt(III)acetylacetonate; 150 mmoles benzene; 5 mmoles 4-nitrotoluene internal standard.

                                      TABLE 3                                      __________________________________________________________________________     RUN 11                                                                                                 % yield NB                                                   Time                                                                              NB   DNB       With respect to                                                                        With respect to                                SAMPLE                                                                               min.                                                                              mmoles                                                                              mmoles                                                                              NB:DNB                                                                              benzene Co(acac).sub.3                                 __________________________________________________________________________     A      10                                                                               2.95 0.00 --   2.0      54.5                                          B      87                                                                               5.64 0.02 282.0                                                                               3.8     104.3                                          C     130                                                                               6.93 0.08 86.6 4.6     128.1                                          D.sup.a                                                                              207                                                                               7.34 0.09.sup.b                                                                          81.6 4.9     135.7                                          __________________________________________________________________________      .sup.a Phenols <0.04 mmoles.                                                   .sup.b The final product had a dinitrobenzene ratio of o:m:p = tr:81:19. 

Table 3 shows the quantities of nitrobenzene and dinitrobenzenes in the various samples taken during the course of the reaction. It appears from the data that after about 2 hours of reaction time, the reaction was approaching its limit based on the limiting quantity of transition metal beta-dicarbonyl compound in the reaction medium. Once again, it can be seen that the nitration process is substantially selective to mononitrobenzene, especially at short reaction times. At longer reaction times the dinitrobenzenes content increases, but the nitrobenzene:dinitrobenzenes ratio is still very high.

EXAMPLE 4

The procedure of Example 2 was followed for Run 12 using as starting materials 17 ml N₂ O₄ liquid; 2.84 mmoles cobalt(III)acetylacetonate; 37.63 mmoles toluene; 5 mmoles nitrobenzene internal standard.

                                      TABLE 4                                      __________________________________________________________________________     RUN 12                                                                                                % yield MNT                                                   Time                                                                              MNT  MNT DNT  With respect to                                                                        With respect to                                 SAMPLE                                                                               min.                                                                              mmoles                                                                              o:m:p                                                                              mmoles                                                                              toluene Co(acac).sub.3                                  __________________________________________________________________________     A      15                                                                               1.20 49:3:48                                                                            --    3.2     42.3                                           B      86                                                                               3.98 50:2:49                                                                            --   10.6    140.1                                           C     135                                                                               4.20 51:1:48                                                                            --   11.2    147.9                                           D     205                                                                               5.15 51:2:47                                                                            --   13.7    181.3                                           E.sup.a                                                                              205                                                                               4.34 56:2:43                                                                            0.0366.sup.b                                                                        11.5    154.2                                           __________________________________________________________________________      .sup.a Sample E is Sample D washed with 5% NaOH.                               .sup.b The final base washed product had the following DNT's ratio:            2,6 DNT:2,5 DNT:(2,3 DNT + 2,4 DNT):3,5 DNT:3,4 DNT = 4:11:31:35:19 with       MNT's:DNT's = 119                                                        

As can be seen in Table 4 the nitration reaction was for the most part substantially completed after about 86 minutes. The reaction was faster than those for benzene and again provided product yields that exceeded the starting quantity of cobalt(III)acetylacetonate. The nitration reaction readily afforded mononitrotoluenes (MNT) without the production of any substantial quantities of dinitrotoluenes (DNT). The nitration reaction primarily affords mononitrotoluene having ortho or para nitrosubstitution.

Cobalt(III)acetylacetonate was present in Examples 2-4 in limiting quantities. Without wishing to be bound by any particular theory, it appears that the initially rapid nitrations slowed, presumably, as the cobalt(III) cation was consumed in the process, i.e. reduced to cobalt(II).

Advantageously, a useful nitration system would include a step or steps in which the transition metal cation in its lower oxidation state were reoxidized. A desirable system would be one in which nitration takes place in the absence of oxygen, with the metal cation restored to its original oxidation state in a separate stage, such as an external oxidation loop. Appropriate oxidation procedures would include the use of oxygen gas or electrochemical oxidation.

It should also be pointed out that the yield of nitrated product based on the added transition metal acetylacetonate was greater than 100% in Examples 2-4 because acetylacetonate is enhancing the nitration reaction in some unknown manner.

EXAMPLE 5

The procedure of Example 2 was followed for Run 13 using 20 ml N₂ O₄ liquid; 2.45 mmole cobalt(III) acetylacetonate; 11.14 mmole benzene and 0.82 mmoles 4-nitrotoluene. A nitrogen purge at about 10 scc/min was continued through the first 291 min at which time the nitrogen flow was ceased and a subsurface flow of oxygen at about 20 scc/min was initiated and continued through the remainder of the reaction. Table 5 shows the data for samples A-H which were taken at the designated times during the nitration reaction. The data shows that the nitration rate increased appreciably after the flow of oxygen was initiated.

                                      TABLE 5                                      __________________________________________________________________________     RUN 13                                                                                                 % yield NB                                                   Time                                                                              NB   DNB       With respect to                                                                        With respect to                                SAMPLE                                                                               min.                                                                              mmoles                                                                              mmoles                                                                              NB:DNB                                                                              benzene Co(acac).sub.3                                 __________________________________________________________________________     A      20                                                                               0.14 0.001                                                                               140.0                                                                               1.3     5.7                                            B      59                                                                               0.33 0.004                                                                               82.5 3.0     13.5                                           C     130                                                                               0.64 0.010                                                                               64.0 5.7     26.1                                           D     183                                                                               0.86 0.013                                                                               66.2 7.7     35.1                                           E     291                                                                               1.13 0.025.sup.a                                                                         45.2 10.1    46.1                                           F     353                                                                               3.45 0.130                                                                               26.5 31.0    140.8                                          G     380                                                                               4.37 0.170                                                                               25.7 39.2    178.4                                          H     431                                                                               5.52 0.198.sup.b                                                                         27.9 49.5    225.3                                          __________________________________________________________________________      .sup.a o:m:p = 7:64:29                                                         .sup.b o:m:p = 6:65:29                                                         *Subsurface oxygen flow commenced                                        

Without being held to any particular theory or explanation, it is believed that this Example indicates that oxygen passed through the nitration reaction medium apparently oxidizes the "spent" transition metal cation, Co(II), to its higher oxidation state, Co(III), whereupon it could again catalyze the nitration of the aromatic compound along with the acetylacetonate groups.

This Example also suggests that the nitration process may be promoted by the combination of a small amount of a transition metal cation in its higher oxidation state and the presence of oxygen to constantly reoxidize the "spent" transition metal cation from its reduced form M⁺(n-1) to the effective higher oxidation state M^(+n), in the case where the oxidation states of the transition metal are separated by one unit.

EXAMPLE 6

Nitrogen tetroxide (20 ml) was condensed into a reaction flask cooled to 0° C. The green tinted nitrogen tetroxide liquid was oxidized to a rich straw color by bubbling oxygen beneath the surface. The flow of oxygen was stopped and nitrogen was bubbled through the solution for 25 minutes in Run 14 and for the duration of the run in Runs 15-18 to remove any dissolved oxygen.

Benzene was added to the liquid nitrogen tetroxide together with a small amount (1-2 mmoles) 4-nitrotoluene as an internal standard for later gas chromatographic analysis. To this solution was added the metal acetylacetonate [M(acac)]. The reaction was maintained at about 0° C. Samples were removed periodically and mixed with an equal volume of methylene chloride for gas chromatographic analysis.

Table 6 shows the amount of benzene and metal acetylacetonate used in each example and the resultant data.

                                      TABLE 6                                      __________________________________________________________________________                                        % Yield NB                                     M(acac)                                                                              Benzene                                                                             Sample                                                                               NB   DNB       with respect                                                                          with respect                         Run                                                                               mmoles                                                                               mmoles                                                                              time (min)                                                                           mmoles                                                                              mmoles                                                                              NB:DNB                                                                              to benzene                                                                            to M(acac)                           __________________________________________________________________________     14 Fe(acac).sub.3                                                                       19.8 30    1.13 <0.05                                                                               --   5.7    51.1                                    1.85       70    3.20 <0.05                                                                               --   16.2   173.0                                15 Co(acac).sub.3                                                                       19.8 8     0.23 0.0008                                                                              298.1                                                                               1.2    11.6                                    1.98       38    2.47 0.08 31.7 12.9   128.8                                              68    3.1  0.14 22.1 16.4   163.6                                16 Mn(acac).sub.3 *                                                                     17.54                                                                               7     0.48 0.007                                                                               58.3 2.8    --                                      >2.0       58    1.16 0.025                                                                               46.4 6.8    --                                                 98    2.27 0.16 12.6 14.4   --                                   17 Cu(acac).sub.2 *                                                                     17.49                                                                               13    0.44 0.0  --   2.5    --                                      >3.8       55    1.20 0.017                                                                               70.6 7.0    --                                                 116   2.35 0.11 21.4 14.3   --                                   18 Ce(acac).sub.4                                                                       17.44                                                                               7     0.19 0.0  --   1.1    12.3                                    1.54       44    1.43 0.0  --   8.2    92.9                                               91    1.64 0.024                                                                               68.3 9.5    108.1                                __________________________________________________________________________      *Part of the M(acac) ignited upon contact with N.sub.2 O.sub.4 vapors.   

Runs 14-18 show that acetylacetonate compounds of iron(III), cobalt(III), manganese(III), copper(II) and cerium(IV) promote the nitration of benzene in liquid N₂ O₄ to yield substantially mononitrobenzene and only minor amounts of dinitrobenzenes.

EXAMPLE 7

The course of a reaction can often be significantly altered by solvents. Furthermore, solvents with very different dielectric constants may have different effects. With this in mind, the following runs were made to ascertain the effect, if any, of a co-solvent.

For Runs 19-24 of this example of procedure of Example 2 was followed using the reactants, co-solvents and quantities as set forth in Table 7. The metal acetylacetonate was added to the co-solvent and this mixture was added to the liquid N₂ O₄.

                                      TABLE 7                                      __________________________________________________________________________              Co-Solvent                                                                               M (acac)                                                                             Aromatic Cpd.                                                                          % NB.sup.a with respect                       RUN N.sub.2 O.sub.4 (ml)                                                                (ml)      (mmole)                                                                              (mmole) to metal (acac)                               __________________________________________________________________________     19  20   nitromethane                                                                             Co(acac).sub.3                                                                       benzene 14                                                     20        15.99 50                                                    20  20   nitromethane                                                                             Fe(acac).sub.3                                                                       benzene 140                                                    14        10.04 50                                                    21  .sup. 10.sup.b                                                                      nitromethane                                                                             Li(acac)                                                                             benzene 129.9                                                  20         2.44 17.44                                                 22  20   nitromethane                                                                             Co(acac).sub.3                                                                       benzene 33                                                     30         7.61 50                                                    23  20   cyclohexane                                                                              Co(acac).sub.3                                                                       benzene 77                                                     30         4.63 50                                                    24  20   methylene chloride                                                                       Co(acac).sub.3                                                                       benzene 14                                                     20        15.99 50                                                    __________________________________________________________________________      .sup.a No dinitrobenzenes (DNB) were detected. The DNB yield, if any, was      estimated to be less than 0.1% of the molar quantity of starting metal         (acac). No phenolics detected; estimated to be <0.4%                           .sup.b Plus 10 ml nitromethane.                                          

As can be seen from Table 7, for the cobalt(III) acetylacetonate the use of a co-solvent, such as nitromethane in Example 22, cyclohexane in Example 23 and methylene chloride in Example 24, limited the conversion to about 33%, 77% and 14%, respectively. In contrast, the use of metal (acac) without an organic co-solvent in Example 2 provided high relative yields of nitroaromatic product, with the reaction rate falling off approximately after a stoichiometric quantity of product was formed.

The most favorable result was Run 20 using iron(III)-acetylacetonate/nitromethane which yielded mononitrobenzene (NB) at about 1.5 times the stoichiometric quantity of starting metal (acac) before being terminated.

The iron(III)acetylacetonate used in Run 20 was a free-flowing powder. However, when contacted by NO₂ fumes the material quickly agglomerates. It therefore cannot be added easily to the N₂ O₄ medium without its first being dissolved in a co-solvent.

Significantly and unexpectedly, only occasional traces of phenolic products were observed in the Runs; on the basis of detection limits, the phenol:aromatic ratio was estimated to be less than 0.4%.

The second significant result is that the degree of dinitration in a reaction medium comprising a great excess of liquid N₂ O₄ without a co-solvent is small. Most surprisingly, however, is that dinitration of benzene unexpectedly appears to be essentially eliminated with the addition of a co-solvent. Estimated detection limits suggest thhe dinitrobenzene:nitrobenzene ratio (Runs 19-24) to be less than 0.1% when an organic co-solvent is used.

STATEMENT OF INDUSTRIAL APPLICATION

The invention provides a method for the nitration of aromatic hydrocarbons by beta-dicarbonyl compounds in a nitrogen tetroxide reaction medium. Benzene can be nitrated by the inventive process to nitrobenzene which is used commercially in the manufacture of aniline, a valuable starting material for the manufacture of dyes, medicinals, resins and varnishes. 

We claim:
 1. In a process for the nitration of an aromatic hydrocarbon, the method which comprises nitrating the aromatic hydrocarbon in a liquid reaction medium consisting essentially of nitrogen tetroxide, optionally an inert organic solvent and an effective amount of a beta-dicarbonyl compound which can assume a "W" configuration with a carbonyl group at each apex of the "W", at a temperature sufficient to effect nitration.
 2. The invention of claim 1 wherein the beta-dicarbonyl compound is an acid anhydride.
 3. The invention of claim 2 wherein the acid anhydride is acetic anhydride.
 4. The invention of claim 1 wherein the beta-dicarbonyl compound is a beta-diketone, a beta-ketoacid, a beta-ketoester, a beta-diacid or a beta-diester.
 5. The invention of claim 4 wherein the beta-dicarbonyl compound is ethyl acetoacetate.
 6. The invention of claim 4 wherein the beta-dicarbonyl compound is acetylacetone.
 7. The invention of claim 4 wherein the beta-dicarbonyl compound is a metal acetylacetonate.
 8. The invention of claim 7 wherein the metal acetylacetonate is an alkali metal or alkaline earth metal acetylacetonate.
 9. The invention of claim 7 wherein the metal acetylacetonate is lithium acetylacetonate.
 10. The invention of claim 7 wherein the metal acetylacetonate is a transition metal acetylacetonate.
 11. The invention of claim 10 wherein the transition metal acetylacetonate is an acetylacetonate of a first row transition metal of the Periodic Table.
 12. The invention of claim 11 wherein the transition metal is titanium, vanadium, chromium, manganese, iron, cobalt, nickel or copper.
 13. The invention of claim 1 wherein the nitration reaction is performed at a pressure sufficient to maintain the nitrogen tetroxide in the liquid state.
 14. The invention of claim 1 wherein the reaction is maintained at a temperature from about -11° to 21° C. and atmospheric pressure.
 15. The invention of claim 1 wherein the reaction is maintained at a temperature above about 21° C. and superatmospheric pressure.
 16. The invention of claim 1 wherein the reaction medium comprises nitrogen tetroxide and an inert organic solvent.
 17. The invention of claim 1 wherein the reaction medium comprises an inert organic solvent and liquid nitrogen tetroxide.
 18. The invention of claims 16 or 17 wherein the organic solvent is a nitroaliphatic, nitroaromatic, acyclic aliphatic, cyclic aliphatic or halogenated aliphatic.
 19. The invention of claim 18 wherein the organic co-solvent is nitromethane, cyclohexane or methylene chloride.
 20. The invention of claim 1 wherein the aromatic hydrocarbon is benzene or toluene.
 21. In a process for the nitration of an aromatic hydrocarbon, the method which comprises nitrating benzene in a reaction medium consisting essentially of liquid nitrogen tetroxide and an effective amount of a beta-dicarbonyl compound capable of assuming a "W" configuration with a carbonyl group at each apex of the "W", at a temperature sufficient to effect nitration and a pressure sufficient to maintain the nitrogen tetroxide as a liquid.
 22. In a process for the nitration of an aromatic hydrocarbon, the method which comprises nitrating toluene in a reaction medium consisting essentially of liquid nitrogen tetroxide and an effective amount of a beta-dicarbonyl compound capable of assuming a "W" configuration with a carbonyl group at each apex of the "W", at a temperature sufficient to effect nitration and a pressure sufficient to maintain the nitrogen tetroxide as a liquid.
 23. The invention of claim 4 wherein the beta-dicarbonyl compound is a metal beta-dicarbonyl compound.
 24. The invention of claim 23 wherein the metal beta-dicarbonyl compound is a transition metal beta-dicarbonyl compound.
 25. The invention of claims 10 or 24 wherein the nitration reaction is performed in the presence of oxygen.
 26. The invention of claims 10 or 24 wherein the nitration reaction includes the step of reoxidizing the spent transition metal cation. 